Combine 1/2 cup water with soy sauce in a small saucepan over high heat. Add onion, ginger and garlic. Bring to boil, reduce heat and simmer for 10 minutes. Remove onion, ginger and garlic pieces with a slotted spoon (discard). Stir in sugar, juice and vinegar. Combine cornstarch and 1/4 cup water in a small bowl or cup. Add mixture to pan and stir while bringing to a boil. Once sauce has thickened, remove from heat.